Post-tensioned trunnion anchor rods are used to secure large devices, such as gates on dams, to structures. To firmly secure the devices to the structure, tension is applied to the trunnion rods, and the individual rods are locked off to maintain the tension load within the rod. See, U.S. Pat. No. 2,705,871 to Kenigsberg and U.S. Pat. No. 2,871,664 to Dobson. For large structures, such as dams, a large amount of tension (typically 60% or more of its ultimate strength) is applied to the rods creating a potentially dangerous situation. At least 100-200 dams in the United States use post-tensioned trunnion rods for gate anchorages. Trunnion rods are predominately made of metal that is designed to operate at a specific tension level. Increased tension, in addition to long-term environmental stresses in the rods creates a dangerous situation where the rods may fail in a brittle and abrupt manner before the end of life expectancy date.
Since 1960 many dams were built using post-tensioned trunnion rods. At the time the dams were erected, and until 2010, cost-effective and safe technology to determine the amount of tension in each rod was not available, and premature rod failures due to over-tensioning have occurred on several dams since the 1970s. Because of the importance of these trunnion rods, and the aging of the inventory of rods currently in dams, dam owners and operators are beginning to regularly test the tension in each rod and the integrity of the rods using recently-developed non-destructive techniques that are safe and cost efficient.
However, even with the testing, there is currently no safe or efficient procedure to remove excess tension in trunnion rods. In a dam environment, an abrupt failure of a post-tensioned trunnion rod can result in the trunnion rod being expelled from the dam at high rate of speed. Further, the costs involved with removing and replacing trunnion rods, or otherwise reinforcing gate anchorages on dams are very high. Therefore, a need exists for a method of removing excess tension from post-tensioned trunnion rods to increase the useful life of installed trunnion rods.